Doxygen: rename all .dox files to end with .md

Using a standard ending here will let other tools that expect
markdown understand our output here.

This commit was automatically generated with:

   for fn in $(find src -name '*.dox'); do \
      git mv "$fn" "${fn%.dox}.md"; \
   done

1 files changed, 9 insertions, 0 deletions

diff --git a/src/lib/fs/lib_fs.md b/src/lib/fs/lib_fs.md
new file mode 100644
index 0000000000..3b5b0ac7d5
--- /dev/null
+++ b/src/lib/fs/lib_fs.md
@@ -0,0 +1,9 @@
+@dir /lib/fs
+@brief lib/fs: Files, filenames, directories, etc.
+
+This module is mostly a set of compatibility wrappers around
+operating-system-specific filesystem access.
+
+It also contains a set of convenience functions for safely writing to files,
+creating directories, and so on.
+